Melting of permafrost threatens homes and roads.

Global warming could melt almost all of the top layer of Arctic permafrost by the end of the century. Scientists say the thaw would release vast stocks of carbon into the atmosphere, threaten ocean currents and wreck roads and buildings across Canada, Alaska and Russia.

David Lawrence, a climate scientist with the US National Centre for Atmospheric Research in Boulder, Colorado, said: "There's a lot of carbon stored in the soil. If the permafrost does thaw, as our model predicts, it could have a major influence on climate." Thawing permafrost is one of several climate "tipping points" feared by environmental experts, because carbon released by melted soil would accelerate global warming. Permafrost makes up about a quarter of land surface in the northern hemisphere and the upper layer is believed to hold at least 30% of the carbon stored in soil worldwide.

Dr Lawrence said: "In terms of its impact on the global climate, I don't see how it can be good news, but just how bad it is is unclear. It's very difficult to see how we can halt it. We may be able to slow it down."